1. Field of the Invention
The present invention relates to a space hopping receiving system that is effective at the place where the electric field strength largely varies due to multipath. Particularly, the present invention relates to a received radio-waves estimation method suitable for estimating interference waves from received radio waves in the frequency hopping system, which is one of spread spectrum communication systems.
2. Description of the Prior Art
Propagation characteristics in land mobile communications greatly depend on buildings around a receiving spot, on mobile velocities, on location of the receivers, and so on. Particularly, because reflected waves or scattered waves from ambient buildings during movement affect conditions of multiple propagation paths (multipath) and many waves via different communication paths interfere with each other, complicated standing waves occur. This largely changes the electric field strength of a received radio wave and changes randomly the phase thereof.
For example, FIG. 6(a) is a computer simulation of a received electric field strength which changes in a two-dimensional plane of 2 m×2 m (400 points) under conditions of multipath, where the vertical axis represents an field electric strength with respect to the x-y plane. FIG. 6(b) is a computer simulation of phase changes of the received field electric strength, where the vertical axis represents phase changes of received radio waves.
In the case of a 12.6 cm (in wave length (λ)) (i.e. 2.4 GHz) band radio wave, as shown in FIGS. 6(a) and 6(b), the phase and electric field strength largely vary even when a mobile body moves in a narrow space.
In order to receive radio waves of a field electric strength against the severe fading phenomenon, a variety of receiving systems have been used commercially and practically. Of those receiving systems, the diversity receiving system is well known as the most effective receiving system.
In the receiving system shown in FIG. 7(a), two antennas A1 and A2, spaced apart from one another, receive radio waves of a desired signal. The receiver 22 receives the signal output from the antenna catching the radio waves of higher electric-field strength, via the switch 21. The level detector 23 detects the strength of the captured signal. When the strength of the captured signal exceeds a threshold value, the switch 21 selects the other antenna.
This system can be simply configured by providing one receiver and an antenna changeover switch. However, when a strong interference wave or the type of a received signal is distinguished, a desired radio wave may not be certainly received. Moreover, the problem is that a time delay accompanying with the antenna switching time occurs.
In order to deal with such a problem, the receiving systems shown in FIG. 7(b) is well known. In this system, two antennas A1 and A2 are spatially separated by a distance. A signal received by one antenna is output via the phase shifter 31 and the variable gain amplifier 32. The synthesizer 33 synthesizes the resultant signal and the output signal of the other antenna. The receiver 34 captures the synthesized signal.
This system is a maximum ratio synthesizing system. According to this system, the detector 35 judges the S/N ratio based on the synthesized wave of the received radio wave and controls the variable phase shifter 31 and the variable gain amplifier 32. Thus, the received wave synthesis is carried out while the CNR is being weighted.
As described above, many measures have been considered to deal with the fading. That is, the system, which includes a number of antennas and receivers each for receiving radio waves via the antennas, performs the maximum ratio synthesis based on outputs from the respective receivers. The software antenna recently proposed may be used. However, such systems require an increased number of receivers and a complicated synthesis method. Hence, this makes it difficult to adapt such systems to slim portable receivers. Particularly, in the communication system where, for transmission and reception, spectrum is spread by frequency hopping, since the received frequency itself varies in a specific pattern, the electric field strength greatly changes due to multipath. As a result, it becomes difficult to estimate interference waves and desired radio waves.